In general, image sensors of camera systems are read out by a control device as a function of an ambient brightness using different exposure times to avoid underexposure or overexposure.
During the day, the exposure times are therefore in general very short. However, it may thus occur that only briefly illuminating light sources, in particular pulsed light sources, are no longer continuously detected, in particular when the read-out time is within or below the order of magnitude of the pulse duration of the light source. For example, pulse frequencies of light emitting diodes (LEDs) in camera systems in the automotive field are less than 0.1 ms during the day at approximately 90 Hz to 100 Hz read-out times of the image sensors.
To prevent an LED detected by the camera system from flickering, DE 10 2010 024 415 A1 provides for the image sensor to constantly or continuously detect incident light using a maximal read-out time. A pause time, during which the image sensor does not convert the light incident on it into electrical signals or is not sensitive to light, is selected to be lower than a pulse duration of light pulses to be detected, so that the sensor does not “overlook” a light pulse. An attenuation filter or a diaphragm is provided to ensure that the sensor is not overexposed or that the sensor does not go into saturation, the attenuation filter or the diaphragm having to be adapted according to the lighting conditions (during the day or at night) since at night less light strikes the sensor than during the day.
The disadvantage of this approach is that additional means are required in the camera system to vary the diaphragm or the attenuation filter in each case as a function of the lighting conditions.
DE 10 2005 033 863 A1 shows an image recording system in which, in addition to a camera, a radiation sensor is provided, which ascertains an average brightness level of an image scene and compares it to the brightness of an image recorded by the camera. The radiation sensor also senses pulsed light sources by using a longer read-out time. If a discrepancy arises between the two brightness levels, a warning signal, which indicates to the driver that a fault exists in the display representation, is output; and the exposure phase of the camera and the activation phase of the pulsed light source are synchronized, or areas of the images represented on the display are replaced by areas with accordingly longer exposure.
The disadvantage of this approach is that a further radiation source, which must be synchronized with the camera, is required, whereby the manufacturing complexity and costs increase.
DE 698 08 024 T2 describes a further method and an image recording system for generating a video signal having an expanded dynamic range. Such systems are also known as high dynamic range (HDR) images, in which consecutive images are generated using different parameter settings, in particular using different exposure times, so that subsequently an optimized image may be created from image sections having different brightness levels, for example by taking over dark sections from the longer exposed image and lighter sections from the shorter exposed image. In this way, the consecutive frames of the output image signal may directly be set differently, so that a sequence of different frames is generated. For this purpose, different exposure times are fixedly set and the images are subsequently evaluated.